This invention relates generally to an electrophotographic printing machine, and more particularly concerns removing contaminants from a half-tone image screen operatively associated with a photoconductive member.
In the process of electrophotographic printing, a photoconductive member is charged to a substantially uniform level. A light image of an original document irradiates the charged photoconductive member dissipating selectively the charge thereon in accordance with the intensity thereof. In this way, an electrostatic latent image is recorded on the photoconductive member corresponding to the original document being reproduced. Heat settable particles develop the latent image. These particles are transferred to a sheet of support material, in image configuration. Heat is applied to these particles permanently affixing them to the sheet of support material.
In the process of multi-color electrophotographic printing each of the previous cycles is repeated for a single color. This process requires that the light image be filtered to record an electrostatic latent image corresponding to a single color of the original document. A plurality of different single color light images are created and the resultant single color electrostatic latent images are developed with particles complementary in color to the color of the filtered light image. The particles are then transferred to the sheet of support material in superimposed registration with one another. This multi-layered powder image is then permanently affixed to the sheet of support material by the application of heat thereto producing a permanent color copy of the original document.
Hereinbefore, it has been difficult to create pictorial quality copies in electrophotographic printing machines. The copying machine has great difficulty in forming tone gradations. This problem has been essentially overcome with the utilization of a half-tone screen. This screen produces tone gradations by forming half-tone dots or lines of varying sizes. In the highlight zones, for example, the dots are small. These dots increase in size throughout the intermediate shades until they merge together in the shadow region. In this manner, pictorial quality copies are reproduced in electrophotographic printing machines.
In multi-color electrophotographic printing machines, contaminants such as toner particles and dirt are frequently more numerous than in conventional black and white reproduction machines. This primarily results from the requirement for three differently colored toner particles, i.e., one for each single color electrostatic latent image being developed. These contaminants will frequently accumulate on the screen member degrading copy quality. Thus, it is necessary to clean the screen member periodically to remove excessive contaminants therefrom.
Accordingly, it is the primary object of the present invention to provide an electrophotographic printing machine having an adjustable screen member which is cleaned automatically.